(1) Field of the Invention
The present invention relates to a fixing device and an image forming apparatus including the fixing device. In particular, the present invention relates to a technology applicable to a fixing device to extend the life of a fixing belt that is included in the fixing device and that has a resistive heat layer and electrode layers for feeding power to the resistive heat layer.
(2) Description of the Related Art
As disclosed, for example, in JP patent application publication No. 2007-272223, some conventional image forming apparatuses (such as printers) employ a fixing device that generates heat upon receiving electric current directly applied to a fixing belt that includes a resistive heat layer.
Such a fixing device provides an advantage of energy savings over a fixing device employing a halogen heater as a heat source.
FIG. 8 is a sectional view of a fixing belt included in a fixing device having a resistive heat layer.
As shown in the figure, a fixing belt 500 includes a reinforcing layer 555 and a resistive heat layer 556 laminated on the reinforcing layer 555.
On the outer circumferential surface of the resistive heat layer 556, a pair of electrode layers 559 are disposed each along an edge of the resistive heat layer 556. The electrode layers 559 are made of metal material and act as electrodes for receiving power from an external power supply.
On the outer circumferential surface of the resistive heat layer 556, in addition, a releasing layer 557 is disposed between the pair of electrode layers 559 for helping a recording sheet to be smoothly released.
Note that the resistive heat layer 556 is made of a material having high electrical resistance and therefore generates heat due to Joule heating in response to the passage of electric current.
With the above configuration, by placing the electrode layers 559 into contact with a pair of power feeders 570 connected to an external AC power source 580, a potential difference is produced across the edges of the resistive heat layer 556 to cause an electric current to pass through the resistive heat layer 556.
As a result, the resistive heat layer 556 generates heat, which is used for thermally fusing an image onto a recording sheet.
Unfortunately, the fixing belt 500 having the above configuration has been found to cause local overheating as a result of the passage of electric current for a long period of time. The overheating occurs locally at around contact portions 560 where the edge of each electrode layer 559 closer toward the releasing layer 557 contacts the resistive heat layer 556.
Such local overheating accelerates deterioration of the heated portions as compared with other portions, which ends up reducing the life of the fixing belt 500.
The following are believed to be the causes of the local overheating.
That is, due to the tendency to flow into where the resistance is lower, the electric current fed to each electrode layer 559 from a corresponding one of the power feeders 570 flows into the resistive heat layer 556 through a portion closer to the other electrode layer 559.
As a result, the electric current flowing between each electrode layer 559 and the resistive heat layer 556 concentrates mainly at the contact portions 560 where the edge of each electrode layer 559 closer toward the releasing layer 557 contacts the resistive heat layer 556.
The electric current flowing into the resistive heat layer 556 locally through each contact portion 560 is then distributed in the thickness direction of the resistive heat layer 556 and concentrates again at around the other contact portion 560.
As a result, the current density reaches the maximum at the contact portions 560, which results in overheating at the corresponding portions of the resistive heat layer 556.